The effects of amiodarone on thyroid function tests in 100 patients treated for 6 weeks to 8 years are reported. One patient became thyrotoxic and 10 developed latent or overt hypothyroidism. Twenty-five patients remained clinically euthyroid throughout, but had free thyroxine indices above the normal range. In these patients with apparently anomalous results, total tri-iodothyronine was normal in 19 cases and low in 1; conversely, free thyroxine was high in all 17 cases in which it was measured. Thyrotrophin releasing hormone (TRH) tests were abnormal in 4 of the 13 patients who had the test. Reverse tri-iodothyronine was significantly raised after 2 weeks amiodarone in 5 healthy subjects, but an equivalent amount of iodine in 9 healthy individuals did not significantly affect any of these tests. We believe that these changes are due in part to inhibition of peripheral conversion of thyroxine to tri-iodothyronine with diversion to reversed tri-iodothyronine. Thyroid function tests should be checked once or twice a year in all patients on maintenance amiodarone. Tests indicating hypothyroidism are likely to be clinically relevant, whereas levels of thyroxine suggesting thyrotoxicosis may be misleading and do not usually imply the need to discontinue treatment with the drug.